High pressure metal halide discharge lamps generally comprise a tubular fused silica arc tube containing an ionizable fill and having a pair of main thermionic electrodes in the ends. The electrodes are supported by inleads which include a thin molybdenum ribbon portion extending hermetically through a pinch or press seal in the end of the lamp. Generally a starter electrode is disposed in the arc tube adjacent one of the main electrodes to facilitate starting. In most lamps a discharge can be ignited between the starter and the adjacent main electrode at a much lower voltage than between the two main electrodes, and ignition of the arc between the main electrodes is thereby facilitated. The common practice has been to connect the starter electrode through a current limiting resistor, typically about 40,000 ohms, to the same side of the power supply as the opposite main electrode. This limits the starter current during normal operation to an insignificant percentage of the arc current.
During operation of metal vapor lamps and in particular metal halide lamps containing alkali or alkaline earth metal additives, electrolysis can occur within the press seal between the inleads of the starter and of the adjacent main electrode if an electric potential exists between them. Electrolysis occurs primarily as alkali ion displacement through the silica, and can always occur, irrespective of lamp fill, because high silica glass or fused silica contains minute quantities of alkali metals as impurities. However, it is much greater when an alkali metal, such as sodium in the form of an iodide, is provided as part of the lamp fill. Electrolysis causes the silica to devitrify and leads to cracking of the hermetic seal, or alternately it deteriorates the molybdenum ribbon to the point of failure.
Several schemes have been proposed to overcome the electrolysis problem and the most widely used is that of U.S. Pat. No. 3,226,597 -- Green, which provides a thermal switch in the form of a bimetal element to electrically short circuit the starter electrode to the adjacent main electrode when the lamp reaches normal operating temperature. An alternative proposed by U.S. Pat. No. 3,619,711 -- Freese, provides a semiconductor diode connected between the starter and the adjacent main electrode and poled to prevent the starter from developing a negative potential.
Most metal halide lamps include sodium iodide as part of the fill and sodium ions can migrate by electrolysis through the fused silica wall during operation. As sodium is selectively lost and freed iodine is left behind, the lamp spectrum deteriorates through loss of sodium radiation and the operating voltage may rise, ultimately to the point of lamp failure. When a conventional side rod construction is used for the mount or frame which supports the arc tube within the outer envelope, the radiant emission from the arc tube causes the side rods to emit photoelectrons. Some of these photoelectrons drift to the outer surface of the arc tube, charging it up negatively and accelerating the electrolysis of sodium ions through the fused silica. Sodium loss can be reduced by changing the mount frame or harness to a divided mount construction wherein the mount is in two sections, one extending from the stem and the other extending from the dome end of the outer envelope to the arc tube. However even with the divided mount construction, an appreciable loss of sodium continues to take place.